The invention relates to a viscous coupling for a cooling air fan, especially for the internal combustion engine of a motor vehicle.
From German Publication Specification No. 27 50 289 a viscous coupling for a cooling air fan of a motor vehicle internal combustion engine is known in which a housing carrying the fan blades is rotatably mounted on a drive-input shaft driven about a rotation axis by the internal combustion engine. The housing contains a partition which divides it in the axial direction into a reservoir for a viscous shear fluid and a working chamber. In the working chamber there is arranged a rotor non-rotatably held on the drive-input shaft. The rotor together with the housing or its partition defines at least one shear gap. A temperature-dependently controllable valve device connecting the reservoir with the working chamber controls the flow of the shear fluid from the reservoir into the working chamber. On the working chamber side the partition is adjoined in the region of the external circumference of the rotor by an annular space defined at least partially by the rotor. The partition contains at least one over-flow opening connecting this annular space with the reservoir. A cylindrical roll is guided on the housing for movement in the circumferential direction of the annular space between two stops in relation to the over-flow opening. The roll forms a pump element which on rotation of the rotor pumps the shear fluid back from the working chamber into the reservoir, by reason of the slip. The roll changes its end position in dependence upon the direction of rotation of the rotor. Thus the viscous coupling can be operated in both directions of rotation of the rotor and universally used.
In the known viscous coupling the flow-favoring cylinder form of the pump element has a disadvantageous effect upon the pump action. Furthermore, to accommodate the pump element two noses arranged with spacing in the circumferential direction are provided on the inner jacket of the housing, which form a pocket between them for the accommodation of the pump element and reach to close to the external circumference of the rotor. The noses baffle the shear fluid without increasing the pump effect. Finally, in the acceleration and retardation of the rotor, the inertia forces of the pump element take effect and prevent the flow of shear fluid from being as uniform as possible in all operational conditions.